This invention relates to flowmeters of the vortex-shedding type. More particularly it relates to vortex-shedding flowmeters wherein Karman vortex streets are generated which provide strong signals, a high signal to noise ratio and a predictable, improved Strouhal number.
The operation of a vortex-shedding flowmeter is based upon placing a bluff-body obstacle in the flow path of a fluid, providing regular vortex patterns which are known as a Karman vortex street. The repetition rate at which vortices are shed in a Karman vortex street is a direct function of flow velocity.
The accuracy, however, of a vortex-shedding flowmeter depends on several factors. Karman vortex streets oscillate in a generally sinosoidal wave form and various sensors are known for detecting the frequency or amplitude of this wave, converting the sensed variable into an electrical signal and then through appropriate electrical circuitry converting such electrical signal into a measurement of flow rate or flow velocity.
A major consideration in the design of a vortex-shedding flowmeter is the signal strength and the related signal-to-noise ratio. Noise is generated by turbulent fluid fluctuations unassociated with the Karman vortex street. The strength of the Karman vortex street signal divided by this noise level is the signal-to-noise ratio. Signal strength is directly proportional to the amplitude of the Karman vortex street. The greater the amplitude the less significant will be any random variations in amplitude. By increasing signal strength without a proportional increase in noise, the signal-to-noise ratio is increased providing greater sensitivity to the flowmeter.
Intermittency of the signal occurs however when the Karman vortex street randomly varies in either or both its amplitude or frequency. Much work has been done by others to avoid this intermittency problem which affects accuracy. Rodely in U.S. Pat. No. 3,572,117 found that intermittency could be reduced by using a bluff-body having sharp vortex-generating edges wherein the bluff face of the body has a width d of from 15 to 40% of the inside diameter of the flowpipe and the body has a length l downstream of such sharp edges of from 1.0d to 2.0d.
Another phenomena which affects accuracy is "drop-out" which is a sudden, very brief, random loss of measured signal caused by a momentary weakening of signal or momentary increase in background noise.
Another consideration, however, in the design of a vortex-shedding flowmeter is the Strouhal number which relates to fluid flow according to the equation EQU St.=(fd)/(V) (1)
where St. is the Strouhal number, f is the frequency of the Karman vortex street, d is the characteristic width of the bluff body surface transverse to the direction of fluid flow, and V is the mean flow velocity of the fluid. At low Reynolds numbers the Strouhal number varies with flow velocity, but at higher Reynolds numbers in the range of about 10.sup.4 to 10.sup.6 the Strouhal number is independent of flow velocity. It has been found that, all other variables being constant, the higher the Strouhal number the more accurate the flowmeter because the adverse affects of drop-out diminish. It can be seen from equation (1) that the Strouhal number is directly proportional to the frequency f of the Karman vortex street. Thus, at any constant flow velocity V, the Strouhal number can only be increased by increasing the frequency f of the Karman vortex street or by increasing the width d of the bluff body.
Since the bluff body is an obstacle there is a certain pressure drop in the fluid flowing past it. To increase its width d beyond that optimumly dimensioned for its bluff body performance is counter-productive since it would present more of an obstruction and greater pressure drop.
Thus, increasing the frequency f of the Karman vortex street is the most desirable way of improving accuracy by increasing the Strouhal number.
It has also been found that for any given width d of a bluff body the frequency of the Karman vortex street tends to increase as the length l of the bluff body downstream of the bluff face decreases. Rodely, however, precludes length l from being less than 1.0d without intermittency becoming a problem.
The objective of this invention is to provide an accurate vortex-shedding flowmeter capable of providing a Karman vortex street having an improved combination of signal strength, high signal-to-noise ratio, high frequency and a predictable, improved Strouhal number.